The word "callous" in this context refers to being emotionally insensitive or unfeeling. Since Douglass mentions that his fellow slaves' backs had become "familiar with the bloody lash" and they had become "callous," it implies that they have become unfeeling or indifferent to pain due to their experiences. In contrast, he describes his own back as "yet tender," suggesting that he is still sensitive to pain and suffering.
Based on this analysis, the correct meaning of "callous" from the choices provided is:
unfeeling.